This invention relates to semiconductor devices, and more particularly to manufacture of MOS VLSI transistors.
Molybdenum gate MOS access transistors and molybdenum word lines are employed in high density DRAM arrays, such as the 1-Megabit devices, due to the lower resistance of molybdenum compared to polysilicon. Likewise, it is advantageous to silicide or clad the surface of the source/drain (moat) regions for these access transistors to lower the series resistance of the such regions. However, problems have occurred in attempts to execute both metal-gate and direct-react clad-moat processes on the same devices due to the fact that the chemicals used to strip the unreacted metal in a clad-moat process can attack the metal gate material through defects in the metal-gate encapsulation layers. Theoretically, this problem could be solved by the use of an unreacted titanium-metal stripping-acid which does not attack molybdenum metal. Examples would be dilute hydrochloric acid or dilute sulfuric acid. In practice, it has been found that a selective acid technique based on consideration of the gate and cladding materials does not work for the following reasons: the metal deposited for moat cladding reacts to form a silicide over exposed silicon regions but does not remain to be a pure metal over the oxide regions as desired, due to reaction of the titanium with the siliciding-furnace gases and with the plasma-oxide encapsulation. Dilute hydrochloric acid is not effective in removing the furnace-processed titanium. Dilute sulfuric acid removes the furnace-processed titanium-metal but stops on a conducting interface layer formed at the plasma oxide encapsulation interface.
It is the principal object of this invention to provide an improved method of making VLSI MOS devices, particularly metal-gate clad-moat devices. Another object is to provide a method of etching un-reacted refractory metal from a surface of a silicon slice without undesired etching of encapsulated refractory metal gates.